ALPHA CIR (Alpha Circini). The luminaries (brightest stars) of
some constellations are special. One thinks of Vega, Arcturus, Alpha Centauri. Here is another one,
the brightest "roAp" star of the sky. It does not much look so
special. Circinus, the Compass or
Compasses (not the navigation, but the measuring, instrument), is
a small modern constellation overwhelmed by nearby southern Centaurus, known for Alpha and Beta Cen. Its brightest star, Alpha Cir, is
just mid-third (3.19) magnitude. Because of the peculiar spectrum,
this class A star's subclass is hard to judge. Formally, it is an
"ApSrEuCr" hydrogen fusing dwarf, where the "p" stands for
"peculiar," and the other letters indicate strong enhancements in
strontium, europium, and chromium. A temperature of 8000 Kelvin
suggests class A7. From a distance of only 54 light years, the
star shines at us with a luminosity of 11 times that of the Sun, which leads to a radius of 1.7 solar and
a mass of 1.8 solar. "Ap" stars are characterized by strong,
localized magnetic fields that seem to be arranged in patches on
the stellar surfaces, in which various chemical elements (including
those above) are enhanced by diffusion/elevation processes. No one
really knows why, or whether the magnetic fields are "primordial"
(from birth) or created in the stars themselves, or how they are
generated. The classic case is Alpha-2 Canum Venaticorum (Cor Caroli). Variations in the spectrum
and in brightness give a rotation period of 4.48 days and (from the
projected rotation speed) an axial tilt of 42 degrees. The
magnetic field comes in at about 500 times that of the Sun, really
not all that large. In addition, the star oscillates and varies
quickly with periods of 6.825 and 6.832 minutes (with respective
variations of 0.002 and 0.004 magnitudes, hence "roAp," for
"rapidly oscillating class A peculiar") as well as other periods,
including one of 2.94 hours that may indicate pulsations like those
seen in Delta Scuti. A four month
period may be present as well. Some 16 seconds of arc away lies a
likely companion, an eighth magnitude (8.47) class K5 dwarf (its
mass less than that of the Sun) that orbits at least 260
Astronomical Units away with a period of at least 2600 years. If
at that separation, Alpha Cir proper would see the companion
shining at twice that of our full Moon, while the companion sees
Alpha 130 times brighter.